Internal-combustion-operated, freepiston gas-generators



Marh14,1961 E. s. L. BEALE v 2,974,653-

INTERNAL-COMBUSTION-OPERATED, FREE-PISTON GAS-GENERATORS Filed July 31, 1957 E ATTORNEY INTERNAL-COMBUSTION-OPERATED, FREE- PISTON GAS-GENERATORS Evelyn Stewart Lansdowne Beale, Wraysbury, England,

assignor to Alan Muntz & Company Limited, Hounslow, Middlesex, England Filed July 31, 1957, Ser. No. 675,330

Claims. (Cl. 123-46) The present invention relates to internal-combustionoperated, free-piston gas-generators, that is to say a machine comprising an engine piston working in an engine cylinder and constituting a prime mover operated on the Diesel principle, the piston movements being determined at least mainly by gas pressures acting thereon, and a compressor piston directly coupled to the engine piston and working in a compressor cylinder. Air com.- pressed in the. compressor cylinder is fed to a scavenge air reservoir, usually the engine-case, and thence passes through the combustion chamber of the engine, as scavenge air, and through the exhaust ports of the engine to a driven machine, such as a turbine.

The delivery from the gas-generator is usually controlled automatically in dependence upon the load by variation in the fuel supply to the engine. However, at low values of load difliculty arises in sufficiently reducing the gas delivery because the stroke of the pistons cannot be reduced beyond the point at which the inlet and exhaust ports are uncovered by the engine piston. If excess gas is merely blown off to atmosphere this results in an undesirable reduction of efiiciency at low powers.

The present invention has for its object to provide means for improving the efiiciency of free-piston gasgenerators at low power levels.

According to the present invention there is provided an internal-combustion-operated, free-piston, gas-generator in which a duct connects the scavenge air reservoir, usually the engine-case, directly with the compressor cylinder, a control valve being arranged to open the duct at low power levels and close the duct at higher power levels. The word directly is intended to exclude the case where the connection between the scavenge air reseivoir and the compressor cylinder is through the suction casing which is open to the atmosphere.

The control valve may be a throttle valve operated automatically in dependence upon the power supplied by the gas-generator; for instance it may be operated in dependence upon a control pressure which controls the fuel pump rack (or control member), and which, in turn, may be dependent upon the speed of a driven machine, or in dependence upon the pressure in the scavenge air reservoir.

A small amount of air will flow through the throttle valve from the compressor cylinder to the scavenge air reservoir when the pressure in the cylinder is above that in this reservoir, but this is merely equivalent to increasing the effective area of the delivery valves very slightly.

The gas-generator may be of the form in which two piston assemblies, each comprising an engine and a compressor piston, move in opposite directions, the two engine pistons operating in a single engine cylinder, and the compression strokes of the compressor pistons being the inward strokes thereof which are also the compression strokes of the engine pistons. Connection with the compressor cylinder may be made through the cylinder atent O wall near to the inner end thereof where the suction valves are located. Alternatively the connection may be made through the head plate containing the delivery valves and separating the compressor cylinder from the engine case.

The arrangement described has the disadvantage that although some air will blow back into the compressor cylinder during the re-expansion stroke (that is the outward stroke), thereby doing some work on the compressor piston, air will also blow back during the com pression stroke when the resulting rise in cylinder pressure will require more work from the compressor piston.

An important subsidiary feature of the present invention has for its object to recover some useful work from the expansion of air blown back from the engine case. According to this feature of the invention, the control valve is arranged, at low power levels, to be opened and closed in step with the movements of the compressor piston in such a manner that it open during the first part of each re-expansion stroke and closed during the latter part of each re-expansion stroke. The reexpansion stroke is the stroke opposite to the compression stroke and in the case considered of an inwardcompressing piston the re-expansion stroke is the outstroke.

There is no objection to arranging that the control valve' is also open during the corresponding part of the compression stroke, that is the last part of this stroke. In fact there is some advantage since the delivery valves are open at this time and the passage through the control valve assists the delivery valves and reduces delivery losses.

Ideally the control valve is given a large effective area and is so timed as to be open for such a fraction of the stroke after the inner dead point as to allow the required quantity of air to blow back into the cylinder with only a small pressure difference across the valve. In this way there would be no throttling losses, and the device would behave as if the compressor cylinder had a larger clearance volume, thereby simply reducing its effective suction stroke.

A valve to do this would need to have rather a large eifective area to pass the volume displaced by the piston with only a moderate pressure drop such as 4-5 p.s.=i. In one example the effective area required for the valve in each compressor cylinder is about equal to the equivalent hole area of one of the twelve delivery valves provided for each cylinder. A smaller valve than this, giving a larger pressure drop, would still be effective in blowing back the required volume. This would mean that it would be open for a rather larger fraction of the out-stroke and the losses due to throttling would reduce the energy recovered at the-piston. However, the losses would raise the temperature of the scavenge air and so give some of the benefit obtained by a simple throttle valve.

The control valve may, for example, be arranged to be operated mechanically, hydraulically, or pneumatically. It may be operated from the synchronising gear provided to synchronise the movements of the two piston assemblies. It may be opened for an equal fraction of the stroke before and after the inner dead point, the period of opening being adjusted automatically by a mechanism controlled either directly, or through a servo-mechanism, by the gas delivery pressure or by a control pressure, for instance, as described in the earlier specification referred to. The arrangement is, therefore, such that the valve opens and closes only at low power levels: at medium and high power levels the valve remains closed.

The invention will be described, by way of example, with reference to the accompanying drawing in which:

Figs. 1, 2 and 3 are diagrammatic representations of part of a gas-generator according to the invention, and

Fig. 4 shows a modification of a part of Fig. 1.

Like parts in the several figures have the same references.

Referring-to Fig. 1, a piston assemblycomprises an engine piston and a compressor piston 11 rigidly connected together, the engine piston 10 Working'in an engine cylinder 12 and the compressor piston 11 working in a compressor cylinder 13. A second piston assembly is provided of the same construction as that described'with its engine piston '10 operating in the engine cylinder 12. Only the engine piston of this second assembly is shown. The two piston assemblies are coupled together, so that they reciprocate towards and away from one another in synchronism, by racks 14 and 14 connected to the two compressor pistons and a gear wheel 15 rotatably mounted upon an engine-case 16. The engine-case 16 constitutes a reservoir for scavenge air compressed by the compressor pistons. Thus air is drawn into the compressor cylinder 13 during the outward strokes through valves 17 and compressed air is forced into the enginecase during the inward strokes through valves 18.

The inward strokes of the pistons 10 and 10' compress air in the space between them. At the appropriate moment fuel is. injected (by well known means not shown) into this space and is ignited automatically by the heat of compression. When the pistons are thus driven outwards, they uncover inlet ports 9 through which air stored in the engine-case passes into the combustion space and exhaust ports through which the products of combustion and surplus scavenge air pass to a delivery pipe (not shown) to a machine such as a turbine to be driven.

Air compressed in a cushion 19 during the outward, or firing, stroke serves to provide the return energy needed to drive the piston assembly 10, 11 on its compression stroke.

It may be mentioned that the synchronising mechanism 14, 15 is shown as if within the engine-case only for convenience. It is normally disposed outside the enginecase.

The structure so far described is well known.

In the head plate containing the valves 18, there is provided a throttle valve 20 linked to a piston 21 in a cylinder 22. The space above the piston 21 is connected through a restricted passage 23 to the inside of the engine case 16 and a spring 24 is provided tending to urge the piston 21 upward.

The strength of the spring 24 is so chosen that while i the pressure within the engine-case 16 is low, that is to say at low operating power 'levels, the spring forces the piston 21 upward and opens the throttle valve 20. Thus at these low power levels air flows from the engine-case 16 to the compressor cylinder 13 during the outward strokes of the piston '11 and in the reverse direction during the inward strokes. When the pressure in the engineoase 16 rises above a predetermined value, that is at medium and high operating power levels, the piston 21 is forced downward, maintaining the valve 20 closed continuously.

The restricted passage 23 serves to smooth out fluctuations in pressure in the space above the piston 21.

Referring to Fig. 2 a lever 25 is pivotally mounted upon a piston rod 26 connected to the piston 21. One end of the lever 25 is coupled by a link 27 to the operating lever 28 of the throttle valve 20. The other end of the lever 25 carries a cam follower roller 29 co-operating with a cam 31 on the rack 14. A tension spring 30 tends to maintain the throttle valve 20 closed.

In operation, when the pressure in the engine-case 16 is at a medium or high value and when, therefore, the piston 21 and the fulcrum of the lever 25 are low, the roller 29 is held clear of the 'cam'31. The throttle valve 20=therefore remains closed" continuously:

When the pressure in the engine-case 16 falls below a predetermined value, the fulcrum of the lever 25 rises and the arrangement is then such that during reciprocation of the piston assembly the roller 29 rides upon the cam and the valve 20 is opened. The cam 31 is so disposed that the valve 20 is opened at a suitable distance from the inner dead point position'of the piston assembly and remains open until that same point is reached on the outward stroke.

Since the essential requirement is that the control valve 29 should be open (at low power levels) for a certain time after the inner dead point, it may be arranged to be opened pneumatically by the excess pressure in the compressor cylinder over that in the engine-case (that is in the same manner as a delivery valve), and it may be prevented from closing immediately when this pressure difference is reversed at the inner dead point by a timelag device, such as a dash-dot or a simple restriction in the passage between the compressor cylinder and the control piston. Thus, it is arranged to open later than the delivery valves and also to close later.

Such an arrangement is shown in Fig. 3, wherein the movements of the fulcrum of the lever 25 are controlled, as in Fig. 2, by a piston 21, and the right hand end of the lever 25 is coupled to the throttle valve 20 as described with reference to Fig. 2. In Fig. 3, however, the opening and closing of the throttle valve 20, at low operating powers, that is to say with the fulcrum of the lever 25 in its uppermost position, is effected as follows. A piston 32 is coupled to the left hand end of the lever 25 and works in a cylinder 33 having its lower end in communication with the engine-case through a pipe 34. The upper end of the cylinder 33 is connected through a chamber 35 with the compressor cylinder 13. The chamber 35 is divided by a partition containing a restricted passage 36 and a spring-loaded non-return valve 37.

With the fulcrum of the lever 25 in its uppermost position, when the pressure in the compressor cylinder rises above that in the engine-case, air begins to flow slowly through the restricted passage 36 and when this pressure difference is suificient to overcome the spring loading of the valve 37 this valve opens. The piston 32 is then forced downward and opens the throttle valve 20. When, on the outward stroke, the pressure in the compressor cylinder 13 falls below that in the engine-case, the valve 37 closes but the upward movement of the piston 32 takes place relatively slowly owing to the restricted passage 36.

In the embodiments described with reference to Figs. 1 to 3, the power level at which the throttle valve 20 is opened is determined by the pressure in the enginecase 16. In the modification of Fig. 1 shown in Fig. 4, the power level is determined by the speed of a driven machine, such as a turbine 38. A control device 39 driven by the turbine generates fluid pressure (liquid or air pressure) dependent upon the speed of the turbine and this pressure is applied through a pipe 40 to the upper end of the piston 22 instead of the pressure within the engine-case 16. The pressure in a pipe 41 connected to the pipe 40 may be employed for other control purposes such as the control of the fuel pump rack. Driving gas from the gas-generator is fed to the turnine 38 at 42.

The pistons 21 of Figs. 2 or 3 may be controlled in the way described with reference to Fig. 4.

The various mechanical couplings shown in the figures may, of course, be replaced by pneumatic 0r hydraulic couplings, the latter consisting, for instance, of a driving piston and driven piston connected by an enclosed column of oil.

Although in the embodiments described the control valve 20 -is opened under the control of the air pressure within the engine-case or in dependence upon the speed of a driven machine, it may be arranged to open in dependence upon other characteristics of the gas-generator or of the driven machine. Ways in which this may be achieved will be well understood by those skilled in the a V In carrying out the present invention the arrangements disclosed in the earlier'specification refenred to may be used with only such modifications as are needed in order to apply the present invention.

I claim:

1. In an internal-combustion-operated, free piston, gas-generator comprising an engine cylinder, an engine piston working in said cylinder, a compressor cylinder, a compressor piston coupled to said engine piston and working in said compressor cylinder, at scavenge air reservoir, a port controlled by said engine piston coupling said reservoir to said engine cylinder, a duct coupling said reservoir directly with said compressor cylinder, and a control 'valve opening said duct at low power levels and closing said ductat higher power levels, the

improvement comprising means responsive to the pressure in said scavenge air reservoir falling below a predetermined value to operate said valve to open said duct.

2. In an internal-combus-tion-operated, free-piston, gasgenerator comprising an engine cylinder, an engine piston working in said cylinder, a compressor cylinder, a compressor piston coupled to said engine piston and working in said compressor cylinder, a scavenge air reservoir, a port controlled by said engine piston coupling said reservoir to said engine cylinder, a duct coupling said reservoir directly with said compressor cylinder, and a control valve opening said duct at low power levels and closing said duct at higher power levels, the improvement comprising valve-operating means responsive to the movements of said compressor piston to open said control valve during the first part of the re-expansion stroke of said compressor piston and to close said con trol valve during the latter stroke.

3. A gas-generator according to claim 2, wherein said valve-operating means comprise a cam driven mechanically by said compressor piston.

4. A gas-generator according to claim 2, wherein said valve-operating means comprise means responsive to the pressure difference between said compressor cylinder and said scavenge air reservoir.

5. In an internal-combustionaoperaited, free-piston, gas-generator comprising an engine cylinder, an engine piston working insaid cylinder, a compressor cylinder, a compressor piston coupled to said engine piston and working in said compressor cylinder, a scavenge air reservoir, a port controlled by said engine piston coupling said reservoir to said engine cylinder, a duct coupling said reservoir directly with said compressor cylinder, and a control valve opening said duct at low power levels and closing said duct at higher power levels, the improvement comprising valve-operating means coupling said compressor piston to said control valve and, at low power levels, opening said control valve during the first part of the re-expansion stroke of said compressor piston and closing said control valve during the latter part of said re-ex-pansion stroke.

part of said re-expansion References Cited in the file of this patent UNITED STATES PATENTS 2,200,892 Pescara May 14, 1940 2,429,948 Steiner Oct. 28, 1947 2,435,232 Morain Feb'. 3, 1948 FOREIGN PATENTS 200,465 Australia Dec. (2, 1955 

